1. Field of the Invention
This invention relates generally to a method for transforming a rejected semiconductor product wafer into a recycled EPI-on-substrate wafer suitable for new product fabrication. More particularly the invention relates to the introduction of an epitaxial silicon layer to a heavily doped substrate, the latter of which has been modified by a novel procedure to assure its ability to provide for high performance semiconductor products.
2. Description of the Prior Art
In the early 1970's, when the first semiconductor wafer recycling patents were issued, the silicon wafers used for semiconductor product fabrication were generally created by the Czochralski method to have grown-in oxygen less that 29 ppm. This feature gave the silicon material a high critical shear strength to minimize process stress induced dislocations. Such wafers could be recycled and used again as a replacement material for the more expensive virgin silicon. At times, the less expensive recycled wafers provided higher semiconductor product yields than their expensive virgin wafer alternative.
However after the mid-1970's, the same silicon wafers prepared by the Czochralski method were targeted to have grown-in oxygen at a much higher level, near 32 to 40 ppm. This higher oxygen content contributed to a desired development of crystal lattice imperfections, such as stacking faults and dislocation loops, in the wafer body. Such lattice imperfections in the wafer's interior contribute to higher semiconductor product yields by gettering mobile impurities away from the product's active electronic components near the front wafer surface. This phenomena is called Intrinsic Gettering. Included in the phenomena of Intrinsic Gettering is a recognition that a defect-free zone, or Denuded Zone, must be created near the wafer surface.
The act of recycling a rejected semiconductor wafer by the present industry standard procedure (U.S. Pat. No. 3,923,567) will always remove the narrow Denuded Zone that separates the semiconductor product's active components from the interior lattice imperfections. This removal of the wafer's Denuded Zone makes the resulting recycled silicon wafer unsuitable for future semiconductor product fabrication, unless a method is found to recreate a new defect-free Denuded Zone. This fact has not been addressed in the prior art. A recycled silicon wafer with lattice imperfections at the surface would not yield salable semiconductor products due to the products' having excessive PN junction leakage current. Such prior art recycled wafers are being used successfully as lower valued process monitor wafers.
This invention recognizes the semiconductor industry's requirement for a procedure by which rejected product wafers that originated as epitaxial silicon on low (e.g. less than approximately 1.phi.cm) restivity substrate wafers could be recycled. Such recycled wafers can not be used for process monitor applications because of their low resistivity. The semiconductor industry will be using increasing numbers of these wafers for product fabrication in the future because they are required for high performance applications. It would be desirable to provide a method to recycle a low resistivity substrate wafer back to future use that is highly valued by the semiconductor industry.